This invention relates generally to signal processing equipment and more particularly concerns circuits providing program-dependent response characteristics for signal processing devices.
In the field of audio signal processing equipment, it has been desirable for many years to provide program-dependent response characteristics for signal processors such as compressors, dynamic filters, expanders, noise gates, limiters and virtually any application which involves controlling the dynamic content of an audio signal. The advantages of providing a program-dependent response become obvious under specific dynamic conditions. Optimal response characteristics for some audio signals may require a very fast attack and/or release time and for others may require a very slow attack and/or release time. In dealing with the manipulation of the dynamic content of an audio signal by compression, expansion, limiting or the like, it is very desirable to have a circuit which automatically adjusts its attack and/or release characteristics based on the dynamic nature of the input signal. A long, sustained note from an instrument such as a piano, applied to the input of a downward expander, for example, would require a very long release time-constant to avoid any undesirable pumping effects and provide a smooth decay of the audio signal. Conversely, a short, staccato signal will require a very short release time-constant to avoid any breathing effects, as a release time that is not short enough will allow a burst of noise to pass before the expander actually closes. In compression/limiting applications, it is ideal to provide a circuit which tracks the attack characteristics of the input envelope because it is desirable to have a compressor respond slowly to minor changes in the audio. But if the audio suddenly gets loud, the compressor should have an equally fast attack time-constant.
Such an improvement in the realm of dynamic control over attack and release time-constants would also lend itself to dynamic filtering applications, as the principle side effects of pumping and breathing would be greatly reduced by the increased dynamic sensitivity to the input signal.
Previous attempts to achieve program-dependant release characteristics have fallen far short of ideal results. One compressor currently available boasts a program-dependent release characteristic from 200 milliseconds to one second. Some of the most common compressors currently on the market state in their product literature that their attack time is based on the use of RMS detection in their design, and the design of the RMS detector inherently provides an attack time-constant of 15 mS for 10 dB of gain change, 5 mS for 20 dB of gain change and 3 mS for 30 dB of gain change. While these compressors provide a 5:1 ratio of change, program-dependent release characteristics well beyond this range and even in excess of 1000:1 are desirable.
It is, therefore, an object of this invention to provide an automatic intelligent audio tracking response circuit having program-dependent attack and release characteristics for virtually all dynamic processing equipment. It is also an object of the present invention to provide an automatic intelligent audio tracking response circuit having program-dependent attack and release characteristics which can be used either individually or simultaneously. It is another object of the present invention to provide an automatic intelligent audio tracking response circuit having a ratio of attack time exceeding 1000:1 and a ratio of release time exceeding 1000:1. It is a further object of this invention to provide an automatic intelligent audio tracking response circuit which lends itself to easy implementation for virtually all dynamic processing applications including, but not limited to, downward expanders, linear expanders, compressors, limiters, enhancers and the like. And it is also an object of this invention to provide an automatic intelligent audio-tracking response circuit which is inexpensive, comprised of a minimal number of parts, easy to implement and usable with virtually any type of dynamic processing application.